Of all the deductions woven into the sprawling U.S. tax code, few have been more fiercely guarded than the enormous tax break that lets homeowners deduct the interest they pay on their mortgages.

But as Congress and the White House negotiate the first major rewrite of tax laws in decades, changing the generations-old mortgage-interest deduction -- which costs the government roughly $100 billion a year -- has gone from far-off possibility to part of the conversation.